It almost never fails, even on epic elite. It also SEEMS to work better on some difficult to assassinate mobs than assassinate does, despite having the same DC (supposedly).
I often use it when there is a crowd of mobs. I hit one of em with shadow manipulation, then assassinate the rest 2 at a time while they fight each other.
I also use it as a fire-and-forget assassinate! Its awesome! I highly recommend this.
Consume is also working a lot better, however it is still buggy as all hell. It works REALLY well on some things and seems to always fail on others, even on mobs that shadow manipulation works on all the time.